Volunteers worked all weekend to attach 580 blanket-size, hand-knitted panels to the pedestrian walkways on the downtown bridge, and riggers attached larger panels to the towers.

The planning and permitting started about 18 months ago, said Amanda Gross, 29, who had the idea for the project.

“The county doesn’t have public arts policy. It was a big learning process for everybody,” said Gross, who moved from Atlanta to Pittsburgh about five years ago and soon noticed how crucial bridges are in a city that has three major rivers running through it.

The project was organized by the Fiberarts Guild of Pittsburgh and other local institutions. Gross said knitters from more than 80 Pittsburgh neighborhoods and 120 area townships signed up to help with what the guild calls the nation’s largest “yarn bomb.” The term applies to artists who knit coverings for everyday objects like lampposts, street signs and trees.

Gross said yarn bombing is “really inspirational,” and a good way to bring communities together.

Sherri Roberts, 60, a past president of the guild, said the project started as a “little nugget” of an idea and then “went to town from there.”

The group ultimately had to work not just with designers but with lawyers, architects, structural engineers and riggers to make the yarn bomb a reality. Roberts said the group decided that the panels would contain only colors and designs – no words or political or religious symbols.

Gross said the group even wound up specifying acrylic yarn, since wool absorbs water, is flammable and can be a home to pests.

“Our structural engineer calculated the weight of the yarn, and also when it’s wet,” Gross said. “It was insignificant compared to traffic.”